failur analysis of crane rope

7/30/2019 Failur Analysis of Crane Rope

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Failure Analysis of a Crane Rope

Submitted by:Muhammad Arfan (LS1101201)

Syed Imran Jawaid (LS1105202)


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PROBLEM STATEMENT

A wire rope broke while lifting a load of

reinforcing steel estimated to weigh 2.5 - 3

tones. The precise sequence of events leading to

the failure were not known, but the load did not

drop because the rope jammed in the gap between

sheave and support bracket.


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ANALYSIS STEPS

Case has been analyzed in following three steps;

First Step

Background

Rope Bounce Analysis

Visual Observations

Second Step

Tensile Testing

Rope Damage Analysis

Fractography

Third Step Summary

Conclusion

Recommendations


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FIRST STEP

Background

Rope Bounce Analysis

Visual Observations


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ROPE AND SHEAVE DETAILS

The rope was a general engineering 18 strandnon-spin type designed as 12x7(6/1)/6x7(6/1)

The rope had an inner layer of 6 strands of wires,

with each strand comprising 7 wires wrapped as 6outer wires around 1 inner wire, while the outer

layer is formed by 12 strands of wires wrapped in

the same way.


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ROPE AND SHEAVE DETAILS

The inner core of the wire is fibre. Resistance to rope spin is provided by opposing

twist directions of the inner layer (anticlockwise)

and outer layer (clockwise), whereby the load-

induced torque tends to cancel out.

The rope sheave diameter was 520 mm, and it was

known to have been in service longer than the rope.


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SAMPLING

Three pieces of rope were supplied to assist in thisinvestigation;

These comprised the two broken ends together

with a section of rope taken well away from the

failed ends. The purpose of this latter piece was to

check the load capacity of the rope, via tensile

testing, at the time of failure.


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ROPE BOUNCE ANALYSIS

It is worth considering what the possible effects of ropebounce could be, and how they might contribute to thisfailure. There are different possible effects of rope bounce,

depending on;

whether the rope stays in the groove, or whether it jumps out of the groove

Four possibilities are discussed;

Rope Stays in Groove-No effect

Rope Stays in Groove-Extra Load Induced

Rope Jumps out of Groove-Transient Impulsive Load Induced

Rope Jumps out of Groove-James between Sheave and Boom


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CASE-I (ROPE STAYS IN THE GROVE-NO

EFFECT)

This is incorrect, rope bounce will lead to some levelof transient impulsive load on the rope, which will

add to the dead weight being lifted.


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CASE-II(ROPE STAYS IN THE GROVE-EXTRA LOADINDUCED)

Yes-any bounce of the load would add atransient impulsive force. In extreme cases, this

may double the load being lifted, giving a possible

upper limit to the apparent load of about 5-6 kN.

If the measured breaking load for a piece of wire

rope is around 50 - 60 kN, rope bounce may have

caused the failure to occur. So, validity of this

reason is subject to the tensile testing results. And ifit stands valid, the cause of such a low failure load

would still require investigation.


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CASE-III(ROPE JUMPS OUT OF GROOVE-

TRANSIENT IMPULSIVE LOAD INDUCED)

This might be the result if the rope jumped out ofthe groove and ran freely on the sheave shaft.

However the clearance between sheave wheels and

the carrier is usually quite small. Information on the

measured breaking load in a tensile test will assist

in indicating whether the failure reflected a

transient impulsive load or some more serious

source of loading.


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CASE-IV (ROPE JUMPS OUT OF GROOVE-

JAMES BETWEEN SHEAVE AND BOOM)

This is possible and, if it occurred, would causehigh loads on the rope if the crane operator did not

detect it and there was no trip mechanism installed

to prevent such an event. Such a severe effect of

rope bounce would imply poor lifting practice.

If the measured breaking load in a tensile test is

significantly higher than the stated load being lifted,

and there is no obvious cause of a different ropestate at the fractured region, then this possibility

must be considered.


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Conclusion of Rope Bounce Analysis:

Final endorsement with regard to this activity is

referred to tensile testing results.


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VISUAL INSPECTION

Visual inspection of the rope was performed forfollowing;

Rope specifications

Lubrication and CorrosionAny visible cracks near fracture plane

Location of cracks (if, any)

Any link of cracks with different areas (flattened,round, elliptical etc)


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VISUAL INSPECTION

Rope specifications was same as quoted (RopeSpecification)

As received, rope lubrication was deficient to dry, with

slight corrosion evident on the outside of the rope.

Figure 1 shows the 2 broken ends of the rope.


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VISUAL INSPECTION

Close inspection of the rope near to the fractureplane showed that a number of wires were cracked

in outer and inner strands (Figures 2 & 3)

Fig.2 Fig.3


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VISUAL INSPECTION

Cracks on both inner and inner layers wereassociated with flattened regions on the

wires. Cracking was also observed on wires well

away from this region (Figure 4).

Fig.4


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SECOND STEP

Tensile Testing

Rope Damage Analysis

Fractography


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TENSILE TESTING

Before performing tensile testing of the rope, it was

necessary to establish the original grade and size of thewire rope. This would indicate what degradation ofproperties had taken place over the service life, andprovide an indicator of the severity of service and

quality of maintenance. The only information that the operator could supply,

was that the rope was a 1770 MPa grade.

Thus it was necessary to measure the diameter of wires

near to the break (average approximately 1.5 mm) andthe rope diameter (approximately 21.5 mm).

Information contained in the wire rope manufacturer'stable of properties has been used to find the most likely

original rope diameter and breaking force.

ROPE DIAMETER AND BREAKING LOAD


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ESTIMATION

Manufacturer's data for a range of generalengineering ropes which bracket the measured size

information is given in Table 1.



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ESTIMATION1st Approximation: 26mm diameter

It might initially be thought that the diameter

of both rope and individual wires would have

reduced in service. However, although the rope

diameter would decrease due to bedding in of

the strands, It is unlikely that wire diameter

would change in the absence of significant

plastic deformation



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ESTIMATION2nd Approximation: 24mm diameter

This appears to be the correct specification for the

original load, and the error in measuring wire

diameter is at least 0.02 mm.

Original rope diameter 24 mm, wire diameter 1.5

mm and as manufactured rope breaking force = 332

kN.



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ESTIMATION3rd Approximation: 22mm diameter

Although the rope diameter corresponds to the

value measured, the wire diameter is too small. Inservice, rope stretch and 'bedding-in' would be

expected to reduce the apparent rope diameter, but

the wire diameter should remain unchanged unless

plastic deformation has occurred.


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TENSILE TESTING

The applied load (232 kN) did not represent

complete failure of the rope, but rather fracture of

11 strands (66 wires) out of a total of 18 strands

(108 wires).

Other results are presented in Table below;

The measured breaking force for the section cut

from the rope was 232 kN (approximately 23.2

tonnes). This is some 30% lower than would be

expected.


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ROPE DAMAGE ANALYSIS

Typical damage to the rope is shown in Figure 4.

This damage is analyzed to determine the likely cause

of this damage and to investigate the probable

inference of this for the present failure.

Fig.4


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High Strands

The damage is not due to a high strand although

at first glance the surface of wires in Figure 4 may

seem worn due to the presence of the flat spots.

Close inspection reveals cracks associated with the

flat regions. Little evidence of any severe abrasion is

evident.


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Fatigue- Small Sheave

Although cracks are visible in the wires in Figure

4, which is likely to indicate operation of a fatigue

mechanism, no wires have been bent out of place.

Repeated bending over too small a sheave would

lead to such movement of individual wires.


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Fatigue- Normal Sheave

The observed damage appears to correlate withfatigue damage occurring through bending undernormal loads over a sheave of the correct size. The

flat regions are surface deformation, possiblyoccurring as a result of groove wear during service.

Plastic deformation to regions of the surface ofsome wires has occurred during service. Fatiguecracks have been induced at some of thesehardened regions during bending over the sheave.


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Inference- secondary Effect

This seems to be a correct inference from the tensile testdata. Some fatigue cracking of wires is present in the rope, andthe breaking load has been reduced by some 30%. Nonetheless,this load of 232 kN (23.2 tonnes) is much higher than the load

reportedly being lifted at the time of failure (2.5 - 3 tonnes). Thefatigue cracking is unlikely to have been a direct contributor tothe failure, unless one rope section was more defective thanindicated by this single tensile test.

A 30% reduction in breaking load due to the presence offatigue cracks implies that the mechanism is unlikely to havebeen a primary cause of the failure, unless the section of ropewhich failed was more defective than the test piece.


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Inference -Primary Effect

It is unlikely that fatigue cracking in the wires

was a primary contributory factor in this failure. The

observed breaking load of 232 kN (23.2 tonnes) is

much greater than the load reportedly being lifted. It

is worth keeping in mind, for possible future review,

the possibilities that either the loadwas much greater

than reported or that a particular rope section wasmore defective than implied by the tensile test.


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FRACTOGRAPHY

A number of individual broken wires were cut off the fractured ends

and examined at low magnification using stereo binoculars, and athigh magnification in a scanning electron microscope (SEM).

The total number of wires in all strands was 108, and 20 wires were

selected from the outer strands and 11 from the inner strands.

The wires were de-rusted and ultrasonically cleaned in a de-greasingagent.


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FRACTOGRAPHY

Summary of fracture types observed with number of occurrence is

presented in Table below;

TotalWires

Tensile Cup and cone fracture Flat Twisted FailureFlat semi-elliptical Regions

present

Inner

Strands

Outer

StrandsTotal

Inner

Strands

Outer

StrandsTotal

Inner

Strands

Outer

StrandsTotal

31 1 2 3 0 2 2 9 17 26


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FRACTOGRAPHY

Typical SEM observations of the fracture surfaces are given below at

both a low and a high magnification.

Type 1: Tensile Cup and cone fracture

Low magnification fractograph of cup-and-cone. High magnification fractograph from the central

region of the cup-and-cone fracture.


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FRACTOGRAPHY

Type 2: Flat Twisted Failure

Low magnification image High magnification image


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FRACTOGRAPHY

Type 3: Flat semi-elliptical Regions present

Low magnification image High magnification fractograph from flat semi-

elliptic region shown with arrow.


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FRACTOGRAPHY ANALYSIS

Based on the available information deduced up till this stage, fast

fracture and fatigue are the likely contenders for identification ofsubject failure.

Different fracture modes are considered to determine the

mechanism of failure indicated by these types.(1~3).


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Type-1 Cleavage

This is not cleavage, which is the micro-

mechanism of brittle fracture. The macrograph

shows a tensile cup-and-cone fracture which is

indicative of ductile overload failure. The outerregion is expected to show shear and the inner

region to be ductile.


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Type 1-Microvoid Coalescence

Micro-void coalescence (MVC) is the micro-

mechanism of ductile tensile fracture. In fine scale

microstructures, the voids are quite small and the

initiating inclusions or second phase particles may

be sub-micron in size. The white network in the

picture represents the tear ridges between voids.

Only 3 out of the 31 wires have failed purely by

tensile overload (~ 10%).


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Type 2-Cleavage

Torsional overload failure has occurred right

across the section, except for the small white point

which is the final region of shear. The failure will be

ductile, not brittle.


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MVC is the correct micro-mechanism of fracture,

and it has occurred under a shear stress, thus the

voids are slanted in the direction of applied stress. 2

wires in the outer strands failed by torsionaloverload from the rope experiencing twisting during

failure.


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Small flat semi-elliptic regions are usually a sign of

the presence of a pre-existing defect and would not

generally be expected to have occurred by MVC.


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Type 3-FatigueThis is the most likely explanation for the presence of the small

semi-elliptic regions seen on many of the selected wires. They are

associated with the flattened regions, but fatigue striations cannot be

resolved in this fine scale quenched and tempered microstructure.

There are subtle differences in the appearance of the white ridgesbetween the MVC and the fatigue fractographs.26 wires show

evidence of the existence of small fatigue cracks. Such defects would

reduce the breaking load considerably, but note that the remainder

of the fracture surface shows evidence of ductile fracture, indicatingthat the toughness of the individual wires is high.


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Type 3-Cleavage

The white tear ridges demonstrate the

operation of a ductile mechanism of failure.


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Third STEP

Summary

Conclusion

Recommendations


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SUMMARY

Overall, the strength of the rope was reduced by the presence of fatigue cracks -

this is evidenced by the observed tensile strength of 232 kN compared with themanufacturers stated breaking load of 332 kN.

The observed breaking load is still very much higher than the stated load being

lifted at the time of failure (some 25 - 30 kN). Thus the rope must still have failed

through application of an overload relative to its current strength level. The failure

was not solely due to the presence of fatigue cracks (whose existence is fairlynormal in wire ropes and explains the requirement for regular maintenance and

high factors of safety).

The cause of this overload is not clear, but bouncing of the load might have

allowed the rope to jump from its groove and jam between sheave and boom

during winding. If this state of affairs could exist for a short time undetected, andthe rope winding was continued, a very significant overload could be applied to the

rope.


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SUMMARY

The cause of the fatigue cracks needs clarification. They can initiate as a result of

bending stresses induced by too small a sheave diameter. The recommendeddiameter is 18x rope diameter which equals 432 mm for the resent rope. The

actual sheave diameter was 520 mm, which should have been sufficient. As the

sheave was older than the rope, however, it is possible that wear of the sheave

groove has had an influence. Fatigue cracks can result from deformed surface

regions where ductility thus becomes exhausted, particularly if surface damagefrom abrasion occurs. This would be exacerbated by any decrease in sheave groove

diameter, which could occur by wear during service, and by poor lubrication

practice (which was apparently the case).


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CONCLUSION

The conclusion to be drawn from this investigation is that the

presence of fatigue cracks has lowered the breaking load of the ropeby some 30%. However, the breaking load is still 232 kN, very much

higher than the stated lifting load of 25 - 30 kN. The fractographic

work has indicated ductile fracture in all wires, demonstrating that

the rope metallurgy is up to specification. The most likely cause ofthe fracture seems to be rope jamming between sheave and groove,

probably due to bounce during lifting. The cause of the bouncing is

unknown.

In insurance terms, poor maintenance is not a prime cause of the

failure, which would have been "sudden and unexpected" when it

occurred. Cover should exist for such circumstances.


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RECOMMENDATIONS

Recommendations in the present case are:

Ensure adequate lubrication is maintained in the rope.

Re-groove the sheave at regular intervals and, particularly, when the

rope is replaced by a new one.

Control lifting to avoid bouncing and install detectors which are

activated by rope coming off the sheave.

Monitor condition of rope by surface inspection and tensile testing.

S


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REFERENCES

1. J Llorca and V Sanchez-Galvez (1989) Fatigue and Fracture of Engineering Materials and Structures

Vol. 12 No. 1 pp31-45

2. RE Hobbs and K Ghavami (1982) International Journal of Fatigue April 1982 pp69-72

3. NF Casey and WK Lee (1989) International Journal of Fatigue Vol. 11 No. 2 pp78-84.

4. M Alani and M Raoof (1997) Effect of mean axial load on axial fatigue life of spiral strands,

International Journal of Fatigue Vol. 19 No. 1 pp1-11

5. K Coultate (1997) Magnetic attraction of wire rope testing, Materials World Vol. 5 September 1997

pp519-520.6. K Schrems and D Maclaren (1997) Failure analysis of a mine hoist rope, Engineering Failure Analysis,

Vol. 4 No. 1 pp25-38.

7. MD Kuruppu, A Tytko and TS Golosinski (2000) Loss of metallic area in winder ropes subject to

external wear, Engineering Failure Analysis, Vol. 7 No. 3 pp199-207.

8. J-I Suh and SP Chang (2000) Experimental study on fatigue behaviour of wire ropes, International

Journal of Fatigue Vol. 22 pp339-347.9. M Torkar and B Arzensek (2002) Failure of crane wire rope, Engineering Failure Analysis, Vol. 9 No. 2

pp227-233.

failur analysis of crane rope

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